PYROMETRY 459 couple. This instrument is not fixed focus as is the case with the other types of radiation pyrometer but requires focusing for each sighting distance. The focusing is simply accomplished by means of an ingenious device due to Fefy. Two semi-circular mirrors Fig. 24 (a) inclined to one another at an angle of 5 to 10° are mounted in the thermocouple box, an opening of about 1.5 mm. at the center of the mirrors forming the limiting diaphragm immediately in front of the couple. The observer views by means of the telescope D the image of the furnace formed by the large concave mirror FIG. 24.—Principle of Fery pyrometer. MM and reflected by the inclined mirrors xxf and yy' through a hole in the large mirror. If the image is not correctly focused at 0, the intersection of the two small mirrors, the image appears broken in half as shown by (6). Correct focus is obtained when the two halves of the image are in alignment (c). Thus until correct focus is obtained certain straight lines of the source appear broken in the image. This breaking of a line is illustrated on a magnified and distorted scale by (d). Suppose that the pyrometer were incorrectly focused upon a line source, an arrow, the image falling at position AB instead of at 0. The image of the arrow reflected from the mirror YY' lies at A"B" and that reflected from the mirror xx' at A'B'. To the observer at D the projections of these images appear as two distinct arrows thus —»—>. As the pyrometer is brought nearer into the correct focus by turning the pinion screw and thus moving the large concave mirror in the direction OD the points Pr and P" of the reflected images move along the lines P'O and P"0 coinciding at 0 when the correct focus is obtained. The two arrows are then superposed forming a single image. For the measurement of very high temperatures, usually above 1,500°C., the cover to the front of the telescope is provided with a sectored opening which may be adjusted to reduce the radiation falling upon the receiver by any definite amount, and in this manner the upper temperature range of the instrument is practically unlimited. This adjustment is made by the manufacturer and should not be altered. For the lower scale range the cover is open. The readings with a Fery pyrometer when properly focused, neglecting secondary errors discussed later, are independent of the sighting distance, as is the case with the fixed focus radiation pyrometer.1 The image of the source, as viewed through the small telescope, must cover completely the limiting diaphragm to the thermocouple. This diaphragm appears as a black circular area shown at the center of the field in Fig. 24 (6) and (c). An excellent rule to follow is to sight at such a distance that the 1 See Bureau of Standards Sci. Paper No. 250, p. 97, for the geometrical demonstration of this fact.